Ferric Sulfates on Mars: Mission Observations and Laboratory Investigations

Fe-sulfates on Mars: Sulfate is one of the major types of secondary minerals found on Mars by recent missions, which reinforced its importance for Mars science. Sulfates are excellent hosts of water, the sinks of acidity, and maybe the most active species in the past and current surface/near-surface processes on Mars [1]. Although Mgand Ca-sulfates were identified with wide vertical and horizontal spread in Valley Marineris, Maridiani Planum, and at North Polar Region [2,3,4,5], Fe-sulfates have rarely been observed from orbiters. A few occurrences were recently reported by CRISM team [6], but the overall quantity of Fe-sulfates at the potential occurrences is not comparable with those of Mg-sulfates. Ferric sulfates were found at both landing sites of Mars Exploration Rovers. Jarosite takes ~10 wt% of Meridiani Planum outcrop at Opportunity exploration site [7]. A non-specific ferric sulfate was indicated in the excavated salty soils by Mössbauer spectrometer (MB) [8], for which additional spectral analysis of one site (Paso Robles), using Pancam, MiniTES data, laboratory results, suggests that ferricopiapite should be the major constituent [9]. A two-layer salty soil was excavated by Spirit rover at Tyrone site [10]. APXS data of upper-layer soil (whitish) suggests a Ca ~ SO3 correlation and an elevated Fe content [11]. No APXS and MB measurements were made on the lower-layer (yellowish) soil. A Pancam spectral analysis [12] suggests that ferricopiapite is the major contributor to the yellowish Tyrone soil spectrum, together with jarosite, fibroferrite, rhomboclase, and gypsum. After ~190 sol exposure at the current Mars surface conditions, the Pancam L2/L7 band ratio of yellowish Tyrone soil shows a reduction of > 20%. Preliminary laboratory experiments suggest that dehydration of ferric sulfates (kornelite and copiapite) can be a potential cause; however it only accounts for 7-8% reduction of L2/L7 band ratio [10]. This abstract reports two new hypotheses based on a set of systematic laboratory experiments of ferric sulfates. Experimental study of ferric sulfates: In laboratory, we synthesized eight ferric sulfates and conducted a systematic spectroscopic study on them, including Raman, XRD, MIR and VIS-NIR, reported last year [13]. As the second step of this Mars Fundamental Research project, 150 hydration/dehydration experiments were conducted on five selected ferric sulfates. Our goal was to establish the stability fields and phase transition pathways of these ferric sulfates. In these experiments, water activity, temperature, and starting structure are the variables. Acidic, neutral, and basic salts were used, but no redox state change was intended or observed. The five starting phases were ferricopiapite (Fe4.67(SO4)6(OH)2·20H2O), kornelite (Fe2(SO4)3·7H2O), rhomboclase (FeH(SO4)2·4H2O), pentahydrite (Fe2(SO4)3·5H2O), and amorphous pentahydrite. The hydration/dehydration experiments were conducted at three temperatures (T=50 °C, 21 °C, 5 °C). Ten relative humidity (RH) buffers (from 6% to 100%) were used at each T. Thousands of coupled non-invasive Raman and gravimetric measurements were made at regular time intervals on the intermediate products to monitor the phase transitions. These 150 experiments have been running for almost one year, most have reached the equilibrium. Results and discussion: Among the ferric sulfates that